1. Darth Vader Saves His Son - Return Of The Jedi
The redemption arc to end all redemption arcs, Darth Vader saving Luke Skywalker from Palpatine is one of cinema's most satisfying twists and for good reason.
After the iconic revelation at the end of The Empire Strikes Back, Luke is determined to save his father from the clutches of evil and bring him back from The Dark Side. He goes to him, but ends up engaging in a fierce lightsabre duel that results in him severing his old man's hand.
Refusing to kill Vader and join The Emperor, Luke gets a taste of Palpatine's force lightning as his dad watches on in horror. Deciding that the only one who can maim his boy is him, Vader finally snaps and kills his boss by chucking him down a huge shaft.
Vader's final heroic act solidifies his release from darkness and makes sure that his son's lasting memory of him is a good one. After everything the pair has gone through over the course of the original trilogy, this is the perfect ending to their story.
Yes, it's simple. Yes, it's become cliched. Yes, it's probably been ruined by all the sequels. But, in isolation, this is the greatest example of film enemies uniting.